1. Field of the Invention
The present invention relates to a network synchronization system for synchronizing a master clock in a distributed communication system composed of a plurality of communication devices interconnected by transmission lines, with a reference clock supplied from outside.
In recent data processing and communicaton systems constructed in a company or in a factory, apparatuses for processing data and carrying out communications are decentralized to disperse the functions, loads, and risks. Further, to transmit various kinds of information including, for example, sound signals, image signals, and the like, a fast and large-capacity data transmission equipment with a very high transmission rate is required.
To realize a communication system for carrying out a fast and large-capacity data transmission between a plurality of communication devices, the FDDI (Fiber Distributed Data Interface) standard is now under preparation by the ANSI (American National Standards Institute). In particular, in the FDDI-II, a standardization of a high-speed and large-capacity multimedia backbone LAN (local area network) wherein asynchronous communications such as a packet communication and synchronous communications for transmitting sound signals, image signals, and the like, are carried out, is under preparation.
In synchronous communications system, if the frequencies of the operating clocks on the receiver side and the sender side are not the same, errors such as a clock slip, missing data, or the like, occur on the receiver side due to the frequency difference, and therefore, the frequencies of the operating clocks in all of the communication devices in a communication system must be synchronized with a master clock.
Further, to realize a communication between the communication devices in different communication systems, connected to each other, a phase synchronization between the operating clocks in the communication devices in the different communication systems is required.
Furthermore, if the above communication systems are connected through a public digital communication network, the operating clocks in the above communication devices in the different communication systems must be synchronized with the operating clock of the public digital communication network.
In the above situation, generally, an improvement in the technique for synchronizing all of the communication devices in a communication system with a reference clock supplied from outside, is required.
2. Description of the Related Art
FIG. 1 shows a conventional construction for synchronizing communication devices in a ring-type LAN connected to a public digital communication network, with a reference clock supplied from the public digital communication network.
The LAN shown in FIG. 1 is constructed by connecting a plurality of communication devices 1.sub.1, 1.sub.2, 1.sub.3, . . . 1.sub.i, 1.sub.i+1, . . . 1.sub.n by a transmission line 2. Note, the scope of the present invention does not limit the type of LAN which, for example, can include a star-type, a bass-type, and a mesh-type LAN.
In the construction of FIG. 1, the communication device 1.sub.2, serves as a synchronization control apparatus, i.e., a clock master apparatus. The synchronization control apparatus 1.sub.2, supplies a master clock to all of the other communication devices in the LAN, and contains an oscillator 82 which outputs the master clock, where the phase of the master clock is controllable.
On the other hand, the LAN is connected to the above-mentioned public digital communication network through a time-divisional multiplexing unit (TDM) 122 and a high-speed digital line terminal unit (digital service unit, DSU) 121.
The above-mentioned public digital communication network supplies timing information, i.e., clock information of, for example, 64 kbps+8 kbps, as timings of data transmission, and the clock timing is extracted at the time-divisional multiplexing unit (TDM) 122 as a timing signal of data reception.
Generally, the above LAN is connected to the public digital communication network at one of the communication devices of the LAN. In particular, if the communication device at which the LAN is connected to the public digital communication network, is different from the above synchronization control apparatus, as shown in FIG. 1, the above-extracted reference clock must be drawn over from the time-divisional multiplexing unit (TDM) 122 to the synchronization control apparatus 1.sub.2, through a transmission line 200 provided for supplying only the reference clock.
Although not shown in FIG. 1, the synchronization control apparatus in FIG. 1 includes a PLL circuit wherein the phases of the above reference clock and the master clock output from the above-mentioned oscillator 82 are compared, and the phase of the output of the oscillator 82 is controlled according to the comparison result so that the phase of the master clock is synchronized with the phase of the reference clock.
The above provision of the transmission line 200, however, increases costs. In particular, in a communication system extending over a large area, a distance between adjacent communication devices is often a few kilometers, and therefore, in a communication system wherein the distance between adjacent communication devices is large, the above increase in the cost is considerable.
Furthermore, if a stand-by line is provided against a possible break down of the transmission line 200, the cost is further increased.
In particular, in a completely distributed system, wherein all communication devices can be a clock master apparatus, the above-mentioned reference clock must be supplied to all of the communication devices except the communication device at which the LAN is connected to the public digital communication network. In practice, this is impossible, and therefore, a completely distributed system cannot be constructed due to the above problem.